1. Field of the Invention
The present invention relates to a backlight assembly for a liquid crystal display device and a liquid crystal display device having the same. More particularly, the present invention relates to a liquid crystal display device having a light guiding plate with a novel structure, in which dispersing particles are distributed in the light guiding plate and a light diffusing pattern is formed on a rear portion of the light guiding plate, thereby improving the brightness and the light efficiency of the liquid crystal display device.
2. Description of the Related Arts
A cathode ray tube is generally used as a monitor in a television set, a measuring instrument and an information appliance. Since the cathode ray tube is relatively heavy and voluminous, it does not match with the trend of the miniaturization and light weight requirements of the electronic appliance.
In order to solve the above problem, a liquid crystal display device, which displays information by using the electrical and optical features of liquid crystal filled in a liquid crystal display panel, is developed. The liquid crystal display device not only can match with the miniaturization and light weight requirements of the electronic appliance, but also can reduce the waste of the electric power. Recently, the liquid crystal display device is used in a flat display panel device.
In the liquid crystal display device, a voltage is applied to a predetermined molecule alignment of the liquid crystal so that the molecule alignment of the liquid crystal is changed. By changing the molecule alignment of the liquid crystal, the optical features of liquid crystal cells, such as, birefringence, circumpolarization, dichroism and light scattering features are changed so that an image is variously displayed. That is, the liquid crystal display device modulates the light by using the liquid crystal cells. The liquid crystal display device is classified into a twisted nematic type and a super twisted nematic type. In addition, according to the driving method, the liquid crystal display device classified into an active matrix display type which uses a switching device and a twisted nematic liquid crystal and a passive matrix display type which uses a super twisted nematic liquid crystal.
The liquid crystal filled between a TFT (thin film transistor) substrate and a color filter substrate is a light receiving material which displays information in a screen by adjusting the amount of light incident from an exterior, so the liquid crystal display device requires a separate light source, such as a back light assembly for radiating the light into the liquid crystal display panel.
Since the back light assembly uses a great amount of energy in the liquid crystal display device, the size and the light efficiency of the back light assembly are closely related to mechanical/optical features of the liquid crystal display device. Recently, various kinds of back light assemblies are developed to improve the brightness and the light efficiency of the back light assembly and to reduce the waste of the electric power. The back light assembly is divided into a vertical type and an edge type according to the position of a light source and is divided into a flat type and an inclined type according to the shape of a light guiding plate. Preferably, a back light assembly with the edge type having an inclined light guiding plate is used to reduce the weight, to improve the brightness and to uniformly radiate the light.
Examples of the liquid crystal display devices are disclosed in U.S. Pat. No. 5,502,582 entitled xe2x80x9cLight source cooler for LCD monitorxe2x80x9d, U.S. Pat. No. 5,791,770 entitled xe2x80x9cLight source cooler for LCD monitorxe2x80x9d and U.S. Pat. No. 5,825,614 entitled xe2x80x9cCompact personal computer with a LCD monitorxe2x80x9d.
The amount of the light inputted into the light guiding plate of the back light assembly is different depending on the distance from the light source. In order to solve the above problem, U.S. Pat. No. 5,178,447 discloses the light guiding plate that can improve the brightness of the light and can uniformly distribute the light proceeding in the direction of a LCD panel.
FIG. 1 is a perspective view showing conventional back light assembly for a liquid crystal display device and FIG. 2 is a sectional view of the liquid crystal display device cut along a line xe2x80x9cA1-A2xe2x80x9d portion shown in FIG. 1.
Referring to FIGS. 1 and 2, the back light assembly for a liquid crystal display device consists of a lamp unit 100 and a light guiding unit 200. The lamp unit 100 includes a lamp 110 as light source and a lamp reflector 120 surrounding the lamp 110.
A cold cathode tube is preferably used as the lamp 110. The light generated from the lamp 110 is inputted into a light guiding plate 210.
At this time, since the light from the lamp 110 is omni-directionally radiated, the light radiated to a direction reverse to the light guiding plate 210 have to be reflected towards the light guiding plate 210 by the lamp reflector 120 so as to improve the light efficiency.
The light guiding unit 200 includes a reflection plate 220, the light guiding plate 210, a diffusing sheet and a plurality of light focusing sheets 240.
The light guiding plate 210 is comprised of a transparent plastic material, such as an acryl resin. The light guiding plate 210 is in the form of a panel having an inclined rear surface, a horizontal light output surface, a light incident surface and a side surface. The light from the lamp 110 proceeds towards a liquid crystal display device (not shown) via the light output surface of the light guiding plate 210.
At this time, in order to convert the direction of the light towards the liquid crystal display device, a diffusing ink 212 in the form of a dot pattern is printed on the rear surface of the light guiding plate 210. The diffusing ink pattern 212 turbulently reflects, the light inputted into the light guiding plate 210. Accordingly, the light inputted into the light output surface has an incident angle less than a critical angle of the light guiding plate 210, so that the light is easily guided into a liquid crystal display panel (not shown).
In order to improve the uniformity of the light going to the liquid crystal display panel, the density of the diffusing ink pattern 212 varies depending on where it is formed on the rear surface of the light guiding plate. That is, the density of the diffusing ink pattern 212 is higher as it is more remote from the light source.
The reflection plate 220 is formed on the rear surface of the light guiding plate 210. The diffusing sheet 230 and the focusing sheets 240 are sequentially stacked on the light guiding plate 210.
The reflection plate 220 reflects the light, which is not reflected by the diffusing ink 212, towards the light output surface of the light guiding plate, so the loss of the light can be reduced when it is inputted into the liquid crystal display panel and the uniformity of the light passing through the light output surface of the light guiding plate 210 can be improved.
The diffusing sheet 230 positioned between the light guiding plate 210 and the light focusing sheets 240 refracts the output light, which has a predetermined inclination with respect to the light output surface, towards the liquid crystal display panel thereby improving the front brightness.
The light focusing sheets 240 is positioned between the diffusing plate 230 and the liquid crystal display panel. The light focusing sheets 240 has a plurality of sheets in the shape of a triangular prism having a predetermined arrangement. The light focusing sheets 240 are offset from each other by a predetermined angle. The light focusing sheets 240 reduce the visual angle of the light incident from the diffusing sheets 230 so that the front brightness of the light inputted into the liquid crystal display panel can be improved. Accordingly, the electric power can be saved. Generally, the light focusing sheets 240 include a protecting sheet for protecting the prism sheets. The liquid crystal display panel is installed on the protecting sheet.
However, in the conventional back light assembly, the light source is positioned at one side or both sides of the light guiding plate, so the light inputted into the light guiding plate can be partially leaked through the side surface opposite to the light incident surface. Accordingly, the brightness and the light efficiency of the liquid crystal display panel can be lowered.
Particularly, since the liquid crystal display panel for a monitor has the thickness thicker than the thickness of the liquid crystal display panel for a note book personal computer, the light leakage increases, deteriorating the light efficiency of the liquid crystal display device.
The present invention has been made to solve the problem of the prior art, and accordingly, it is a first object of the present invention to provide a liquid crystal display device that can prevent the light from leaking through a side surface opposite to a light incident surface, thereby improving the light efficiency and the brightness.
A second object of the present invention is to provide a back light assembly having a light guiding plate that can prevent the light from leaking through a side surface of a light guiding plate opposite to a light incident surface.
To accomplish the first object of the present invention, there is provided a liquid crystal display device comprising a back light assembly including a light source for generating a light and a light guiding plate positioned at a side of the light source, a display unit for displaying an image and a container module for receiving the back light assembly and the display unit. The light guiding plate includes a light incident surface for receiving the light from the light source, a side surface positioned opposite to the light incident surface, a light output surface for outputting the light, and a rear surface positioned opposite to the light output surface. The rear surface is formed with a light diffusing portion to uniformly output the light generated from the light source towards the light output surface. The light guiding plate is formed at an inner portion thereof with a light dispersing member for dispersing the light inputted into the light guiding plate.
To accomplish the second object of the present invention, there is provided a back light assembly for a display device comprising a light source for generating a light and a light guiding plate positioned at a side of the light source. The light guiding plate includes a light incident surface for receiving the light from the light source, a side surface positioned opposite to the light incident surface, a light output surface for outputting the light, and a rear surface positioned opposite to the light output surface. The rear surface is formed with a light diffusing portion to uniformly output the light generated from the light source towards the light output surface. The light guiding plate is formed at an inner portion thereof with a light dispersing member for dispersing the light inputted into the light guiding plate.
According to the present invention, the light leaking through the side surface is outputted to the output surface by making contact with the dispersing particles of the light dispersing member, so the light efficiency and the brightness are improved.